1. Field of the Invention
The present invention relates to a communication device used in a wireless communication system, and more particularly, to a communication device of handling a signal transmission on an unlicensed component carrier in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system is developed by the 3rd Generation Partnership Project (3GPP) to satisfy increasing needs of users. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes at least one evolved Node-B (eNB) for communicating with at least one user equipment (UE), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system is an evolution of the LTE system which improves performance at the coverage edge of an eNB, increases peak data rate and throughput by introducing advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmissions/reception, uplink (UL) multiple-input multiple-output (UL-MIMO), licensed-assisted access (LAA) using LTE, etc.
Network operators propose to offload network traffic of the LTE/LTE-A system from a licensed band to an unlicensed band, to ease load of the network traffic. For example, the eNB may provide services to the UE via the unlicensed band.
For transmission in an unlicensed band, listen before talk (LBT) may be needed. To meet the requirement of the LBT, a first clear channel assessment (CCA) operation may be performed within a first time interval, before a transmission is performed. In detail, the eNB may suspend transmission on a downlink (DL) carrier in the unlicensed band for a first time interval to perform the first CCA operation on the DL carrier. In addition, the eNB may suspend transmission for a second time interval and perform a second CCA operation for the second time interval, when the result of the first CCA operation indicates that the DL carrier is busy (e.g., occupied). In this situation, the UE receives no data on the DL carrier during the first time interval and the second time interval. Thus, throughput of the UE is decreased due to the suspensions of the transmissions related to the CCA operation.
Thus, how to handle the transmissions on the carrier (i.e., unlicensed carrier) in the unlicensed band is an important problem to be solved.